Control strategy for gas turbine engine

ABSTRACT

A control strategy for a gas turbine engine which exchanges future lifetime of the engine for present thrust. Gas turbine engines, such as those used in aircraft, sometimes incur damage, as when they ingest birds, or are struck with ballistic objects fired by an enemy. The invention detects the damage, and invokes a control strategy wherein the engine is operated in a more harsh manner, thereby sacrificing a significant part of the remaining lifetime of the engine, in order to obtain thrust currently.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH & DEVELOPMENT

[0001] The invention was made with Government support under Contract No.N68936-99-C-0117, awarded by the U.S. Government. The Government hascertain rights in this invention.

TECHNICAL FIELD

[0002] The invention concerns control systems for gas turbine engines.

BACKGROUND OF THE INVENTION

[0003] In a gas turbine engine used to power an aircraft, malfunctionssometimes occur. While many malfunctions are minor, some are significantenough to reduce thrust of the engine to a large degree. For example, ifthe engine ingests a large bird, damage can occur which significantlycompromises the thrust-producing ability of the engine. As anotherexample, a missile fired by a terrorist can produce similar damage, orworse. As a third example, during take-off, the engine can ingest debrisleft on a runway.

[0004] In such cases, the pilot can take at least two strategies. One isto continue operation of the engine, but at the reduced thrust level.The second is to shut down the engine. The invention provides anotherstrategy for operating a malfunctioning gas turbine engine.

SUMMARY OF THE INVENTION

[0005] In one form of the invention, a system detects damage in a gasturbine engine, as by detecting a lower-than-expected amount of thrust.When the damage is detected, the invention then takes measures toexchange (1) future lifetime of the engine for (2) present thrust.

[0006] For example, the invention may increase a limit on speed of aparticular rotor, which consumes lifetime of the rotor and othercomponents, but produces larger thrust presently. As another example, alimit on turbine inlet temperature may be raised, which again consumeslifetime of components, but produces larger thrust presently.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is a simplified, block-type schematic of a gas turbineaircraft engine within a nacelle 2.

[0008]FIG. 2 shows the engine of FIG. 1, but with a hole 39 in acompressor casing 42.

[0009]FIG. 3 is a flow chart illustrating processes undertaken by oneform of the invention.

[0010]FIG. 4 illustrates one form of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0011] This discussion will present (1) a simplified example of one formof the invention, and then (2) a more general description.

[0012]FIG. 1 illustrates a simplified gas turbine engine 3, which issymmetrical about centerline 4. A fan 6 compresses incoming air 7, anddelivers part 9 to a high-pressure compressor 12. The other part 15 isbypassed, and used to generate thrust. The high-pressure compressor 12further compresses the air, and delivers it to a combustor 18, whereinfuel 21 is delivered and burns, adding energy to the air in the form ofheat.

[0013] The high-energy fuel/air mixture 22 is then ducted to ahigh-pressure turbine 24, which extracts mechanical energy from themixture, and uses that energy to drive the high-pressure compressor 12,through shaft 27. The exhaust 30 of the high-pressure turbine 24 isducted to a low-pressure turbine 33, which extracts further mechanicalenergy, and uses that energy to drive the fan 6, through shaft 36. Theexhaust 35 of the low-pressure turbine 33 is utilized to produce thrust,to the extent possible.

[0014] Assume that, as in FIG. 2, damage to the engine 3 occurs, in theform of a hole 39 in the casing 42 which surrounds the high-pressurecompressor 12. This damage may be inflicted, for example, in wartime bya projectile which strikes the engine.

[0015] With the hole 39 present, compressor efficiency is reduced,because of the loss of air 41. The air 9 delivered to the combustor 18is reduced in mass flow and pressure. As a consequence, thrust of theengine 3 will be reduced.

[0016] The invention detects the reduction in compressor efficiency,that is, makes an inference that damage has occurred. Next, theinvention will take steps to compensate for this reduction inefficiency.

[0017] The invention implements a strategy which will trade (1) thelong-term lifetime of the engine for (2) a temporary present increase inthrust. That is, a reduction in future lifetime of the engine isexchanged for current thrust. Some background principles explaining howthis strategy is possible will be elaborated.

[0018] In brief, many of the components in a gas turbine engine operateunder harsh conditions which deviate greatly from standard, ambientresting conditions of temperature and pressure. The harsh conditionscause wear-and-tear on the components, giving the components a limiteduseful life. The invention operates the engine under harsher-than-normalconditions, to obtain increased thrust, but at the cost of reducing thelifetimes of many of the components.

[0019] To explain these principles in greater detail, it is pointed outthat some components operate in a very high centrifugal force field. Ingeneral, the centrifugal acceleration of a rotating component is(w-squared)×(r), wherein w is angular rotation in radians per second,and r is the distance of the body from the center of rotation.

[0020] If the radius of the high-pressure compressor 12 in FIG. 1 is onefoot, and if it rotates at 10,000 rpm, which corresponds to 167revolutions per second, then the centrifugal acceleration is

(2×PI×167)×(2×PI×167)×(1/sec-squared)×(1 foot),

[0021] or about 1.1 million feet/sec-squared. Dividing this by theacceleration of gravity, 32 feet/sec-squared, gives a g-field of about34,000 G's.

[0022] Thus, this compressor 12 operates under an extremely highG-field: a compressor blade which ordinarily weighs one pound will weigh34,000 pounds in operation.

[0023] In addition, some components are subjected to very hightemperatures, some of which actually exceed the melting temperatures ofthe materials used. Components subject to the latter temperatures mustbe actively cooled, to keep them below the melting temperature. Also,high pressures, sometimes exceeding 400 pounds per square inch, arepresent, which stress materials.

[0024] The high G-fields, the high temperatures, and the high pressuresillustrate three factors which place high stresses on the materialswithin the engine.

[0025] Not only do these three factors, by themselves, generate highstresses, but also the fact that they are applied in a cyclic mannercreates additional stress and wear. That is, when the engine is at rest,these factors are absent. But when the engine is brought into operation,the three factors, previously absent, come into existence and applystresses to the engine.

[0026] The repeated application of stresses, followed by relaxation ofthe stresses, creates fatigue in materials, often resulting in stresscracks. In fact, the usable lifetime of many parts, or at least an outerlimit on their usability, is measured by the number of such cycles thepart has experienced.

[0027] Another factor is the law of physics which states that, astemperature increases, the tensile strength of many materials decreases.Further, components made of such materials may experience a type ofdeformation known as creep. Thus, if a component is operated under highstress at an excessively high temperature, that component may becomepermanently deformed. Further, the creep does not necessarily ariseimmediately, but sometimes after long-term exposure to the stresses justdescribed.

[0028] These stresses, and their cyclic nature, motivate the designersof the gas turbine engines to impose strict limits on the operatingconditions of the engines, to thereby limit the maximal stresses appliedto the components within the engine. For example, the temperature of thegas at point 50, in FIG. 2, called turbine inlet temperature, is held toa certain limit. If the control system (not shown) detects that thelimit is being exceeded, it takes certain measures to reduce it and, ifthose measures fail, may shut down the engine.

[0029] Therefore, to summarize these background principles: in normaloperation, the engine control system maintains many components within agas turbine engine within specific envelopes of temperature, pressure,and rotational speeds. A primary purpose of maintaining the componentswithin their envelopes is to maximize the useful life of the engine.

[0030] The invention makes an exception to this control strategy incertain situations. In the case when a drop in compressor efficiency isdetected, as when hole 39 in FIG. 2 occurs, the invention determinesthat maximizing lifetime of the engine is no longer of prime importance.Rather, obtaining continued thrust from the engine, even for a limitedperiod of time, is assigned higher priority, even if that meansultimately sacrificing significant lifetime of the engine.

[0031] To this end, the invention alters the envelopes just described.The invention alters the limits previously imposed on one, or morecomponents. In this example of a punctured compressor, two limits willbe increased: (1) the limit on high pressure turbine inlet temperaturewill be raised, and (2) that on the speed of the high pressure turbinewill be raised.

[0032] The latter increase will increase mass flow through thecompressor, which will compensate for the mass of air lost through thehole 39.

[0033] The effect of the former increase is not so simple to explain,but can, in general, be explained by the fact that the energy extractedby a turbine is related to the temperature drop across it. If the inlettemperature of the two-turbine system of FIG. 2 is increased, then, ingeneral, those turbines will extract more energy.

[0034]FIG. 3 is a flow chart illustrating a generalized processundertaken by the invention. Block 100 represents the collective outputsof the engine sensors. The modern gas turbine aircraft engine isequipped with an array of numerous sensors, which measure temperaturesand pressures at various locations, and the speeds of the rotors. Manyof the sensors, termed prime sensors, are equipped with back-up sensors,which provide redundancy in the event that the prime sensors fail.

[0035] The sensor outputs are received by the engine control (notshown), as indicated by block 105. Block 110 represents the enginecontrol procedures, or algorithms, which are implemented by the enginecontrol. The Inventors point out that block 110 is a simplification, andthe actual control system, and its processes, are quite complex.

[0036] Engine control systems generally are well known. The controlsystem controls various operating conditions of the engine 3, such as(1) fuel-air ratio, (2) stator vane angle, (3) compressor bleeds, (4)turbine shroud cooling, and so on. These controlled parameters are notshown in the Figures, but are known in the art.

[0037] Branch point 112 in FIG. 3 indicates that the sensor outputs arefed to another destination, namely, block 115, where they aresubsequently utilized by the invention. The outputs received by block115 may include all the outputs of all sensors, or may include onlyoutputs of a subset of the sensors.

[0038] In general, a vector of sensor outputs will be examined. Thevector can be represented as

[0039] (P1, P2, . . . PN, N1, N2, . . . NN, T1, T2, . . . TN)

[0040] herein P represents pressure, N represents speeds, and Trepresents temperatures. The subscripts, namely, 1, 2, and N, indicatethat pressures, speeds, and temperatures at different locations in theengine are measured. Other parameters may be measured as well, such asvibration or deflection.

[0041] Block 120 indicates that the vector is examined. The overall goalis to determine whether the vector indicates that damage to the enginehas occurred. Numerous approaches to making this determination arepossible.

[0042] In one approach, inquiry is made as to whether selectedparameters are within stated limits. For example, if compressordischarge pressure, at cruise conditions, stands below a certain value,then the presence of damage may be inferred.

[0043] In a second approach, the vector is examined for a healthy engineunder normal operating conditions. Then, a selected item of damage isintroduced, and the corresponding vector is examined. Next, a differenttype of damage is introduced, and the process is repeated.

[0044] In effect, this approach derives a signature for each type ofdamage. Types of damage which may be inflicted can include (1) puncturesat various locations, of various sizes, (2) severance of selectedhydraulic, pneumatic, and electrical lines, (3) loss of blades, orpartial blades, in the fan, compressor, and turbines, and (4) others.

[0045] In a third approach, damage is not actually introduced, as in thesecond approach, but the damage is synthesized, in computer models. Verysophisticated computer models have been developed which representoperation of gas turbine engines under many, if not all possible,operating conditions. The damage can be synthesized in the model, andthe resulting signature of the vector can be ascertained.

[0046] In a fourth approach, the damage signature of a vector is reachedby the instinct and experience of the designers of the gas turbineengine.

[0047] Block 125 in FIG. 3 indicates that inquiry is made as to whetherdamage is present. If no damage is inferred, the NO branch 126 is taken,and the process repeats, beginning with block 115. If damage isinferred, the YES branch 127 is taken, and block 130 is reached.

[0048] Block 130 indicates that the type of damage is assessed. Block135 indicates that a type of response is determined, based on the damageassessment. The Inventors point out that blocks 130 and 135 arerepresented as separate, for purposes of explanation. In practice, it islikely that block 130 may be eliminated.

[0049] For example, in practice, each damage signature in the vector isassigned a response. In the compressor-puncture situation describedabove, the signature may be a drop in compressor discharge pressure. Theresponses may be to (1) raise the limit on turbine inlet temperature and(2) raise the limit on speed of the high-pressure compressor 12.However, no actual conclusion that a compressor problem exists isnecessarily reached, or needed.

[0050] Therefore, block 130 is shown in order to illustrate one conceptbehind one form of the invention. But an actual assessment, that is,assignation of a name to the damage, is not strictly necessary. Rather,a direct jump from (1) detection of a deviant vector to (2) a responsefor that deviation can be undertaken.

[0051] Block 140 indicates that the response suitable for the damageindicated by the vector is implemented.

[0052] The processes of FIG. 3 can be implemented in a single computer,or multiple computers.

[0053] Several features and characterizations of the invention will begiven.

[0054] One is that the engine operates according to one control strategyduring normal operation. The invention looks for damage. If damage isdetected, a different control strategy is implemented. One example of adifferent control strategy is to raise the limit on turbine inlettemperature, such as by four percent. Another example is to raise thelimit on core speed, such as by four percent. A third example lies incombining the first and second example. A generalized example is toallow a selected operating parameter of the engine to rise above itsnormal operating value.

[0055] The term normal operation is known in the art. However, it shouldnot be assumed that, for a given engine, normal operation always meansthat similar amounts of deterioration occur during all phases ofoperation. For example, the operating conditions during take-off areharsher than cruise conditions, and greater deterioration occurs attake-off than at cruise.

[0056] One definition of normal operation is that an engine is operatingin normal mode when actual thrust matches demanded thrust. Demandedthrust is generally determined by throttle lever angle. Otherdefinitions are possible.

[0057] A second feature is that the second control strategy involvesaltering the schedule of a controlled variable. The term schedule iswell known, and refers to the fact that a controlled parameter, such asamount of fuel flow, is computed, or scheduled, based on numerous othermeasured parameters, such as rotational speeds, pressures, temperatures,and so on.

[0058] A third feature is that no additional sensors are required. Theexisting sensors on the engine, or a subset of them, whose outputs arerepresented in the vector discussed above, are used. The parametersindicated by those sensors are used to infer the presence of damage, andselect, or modify, the control strategy.

[0059] A fourth feature is that a library of numerous possible damagesignatures of the vector delivered to block 115 in FIG. 3 is maintained.When a damage signature is detected, a malfunction is declared. Thedeclaration can take the form of informing the pilot of the presence ofthe malfunction and, optionally, of the identity of the malfunction. Theidentity is associated with the damage signature at the time of creationof the signature.

[0060] A fifth feature is that, in normal operation, the controloperates the engine so that no parameters, such as pressures,temperatures, and speeds, exceed pre-defined limits. The inventionmaintains (1) a library of damage signatures, each corresponding to aspecific damage condition, and (2) a control strategy for eachsignature, and thus for each damage condition. The invention comparesthe currently derived vector of operating parameters with the libraryand, if a match is found, implements the control strategy correspondingto the matched vector.

[0061] In one embodiment, the selected control strategy causes at leastone operating parameter to exceed its previous pre-defined limit. Inanother embodiment, the selected control strategy causes at least onecontrol schedule to change. In a third embodiment, the selected controlstrategy causes both the changes described in the preceding twosentences.

[0062] A sixth feature is that the invention monitors a set ofparameters which are used by a control system to control operation ofthe engine. If the monitoring indicates that a predetermined event hasoccurred, such as a specific type of damage, then the invention moves alimit on temperature or speed away from its normal operating position,and continues operation of the engine.

[0063] A seventh feature is that a gas turbine engine is operated havinglimits on (1) turbine inlet temperature and (2) speed of a high-pressureturbine. Selected parameters are monitored and, if the parameters, or asubset of them, reach a predetermined state, then one, or both, of thelimits are raised.

[0064] In one embodiment, the predetermined state is that engine thrustis less than 95 percent of demanded thrust.

[0065] In another embodiment, the limit on turbine inlet temperature israised by 2.5 percent.

[0066] In another embodiment, the limit on turbine speed is raised by 4percent.

[0067] An eighth feature is that a gas turbine engine is run in itsnormal mode. The invention monitors the engine and, if a predeterminedevent is detected, such as a specific type of damage, then the controlsystem causes the engine to run at the maximum power available for 30minutes, with no regard for damage inflicted on the engine during thattime. That is, it is acceptable to destroy the engine over that30-minute period, provided the maximum power available is obtained.

[0068] The preceding example stated that the engine was run at maximumavailable power for 30 minutes. The duration of running the engine canbe measured by another parameter, namely, extent of fuel supply.

[0069] For example, the engine 3 is contained in a vehicle, such asaircraft 300 in FIG. 4. The aircraft 300 carries fuel in one or moretanks 305. Computer hardware and software, represented by block 330,undertake the processes described herein. The library of referencesignatures described above is contained in block 330. Block 335represents hardware and software implementing the ordinary enginecontrol system described above. It is understood that blocks 330 and 335need not be completely separate as indicated, but that high degrees ofintegration between them can be implemented.

[0070] If a malfunction or damage as described herein is detected, thenthe remaining useful lifetime of the engine 3 is consumed before thesupply of fuel in tanks 305 is exhausted. Alternately, the engine is runat maximum available power until the supply of fuel is exhausted.

[0071] A ninth feature is that a gas turbine engine is run in a normalmanner, in accordance with a set of control algorithms. A set of theoperating parameters is obtained, and examined by a pattern recognizer.Pattern recognizers are known in the art. The pattern recognizerexamines the set of operating parameters, and looks for a patternindicating a problem has occurred in the engine. The pattern recognizermay do this by comparing the set of parameters with stored signatures ofparameters, each of which indicates a specific problem. If a problem isfound, the set of algorithms is altered, and the engine is continued torun.

[0072] Numerous substitutions and modifications can be undertakenwithout departing from the true spirit and scope of the invention. Forexample, the invention has been framed in terms of an aircraft engine.However, land-based engines, such as those used in armored tanks, canutilize the invention, as well as sea-based engines, such as those usedin ships.

[0073] What is desired to be secured by Letters Patent is the inventionas defined in the following claims.

1. A method, comprising: a) operating a gas turbine engine according toa first control strategy; b) looking for damage to the engine; and c) ifdamage is found, operating the engine according to a second controlstrategy.
 2. Method according to claim 1, wherein the second controlstrategy allows an operating parameter of the engine to exceed itsnormal operating value.
 3. Method according to claim 1, wherein thesecond control strategy allows an operating parameter of the engine toexceed its normal operating value by four percent.
 4. Method accordingto claim 1, wherein the second control strategy changes the schedule ofa controlled variable.
 5. A method, comprising: a) delivering measuredoperating parameters in a gas turbine engine to a control which controlsengine operation; b) using some, or all, parameters to infer damage tothe engine; c) if damage is inferred, taking one or both of thefollowing actions: i) notifying an operator of the engine; ii) alteringcontrol strategy of the engine.
 6. A method of operating a gas turbineengine, comprising: a) maintaining a library of reference signatures ofoperating parameters, each of which represents a respective damagecondition in the engine; b) monitoring current operating parameters, andcomparing them with the library; and c) if the current operatingparameters match a reference signature, declaring a malfunction.
 7. Amethod, comprising: a) controlling a gas turbine engine in a manner thatno operating parameters exceed pre-defined limits; b) examiningoperating parameters for a damage signature indicating the existence ofdamage to the engine; and c) if a damage signature is found, controllingthe engine so that at least one of the operating parameters exceeds itslimit.
 8. A method, comprising: a) controlling a gas turbine engine in amanner that no operating parameters exceed pre-defined limits; b)maintaining a library of reference damage signatures, wherein eachsignature indicates a different damage condition of the engine; c)maintaining a collection of control strategies, each strategycorresponding to a different damage condition; d) deriving a vector ofoperating parameters from the engine; e) based on a comparison of thevector with the library, i) determining whether a damage conditionexists, and ii) if so, selecting a corresponding control strategy andimplementing it.
 9. Method according to claim 8, wherein the selectedcontrol strategy causes at least one operating parameter to exceed itslimit.
 10. Method according to claim 8, wherein the selected controlstrategy changes the control schedule of at least one controlledvariable.
 11. Method according to claim 8, wherein the selected controlstrategy causes i) at least one operating parameter to exceed its limit,and ii) a change in the control schedule of at least one controlledvariable.
 12. A method, comprising: a) operating a gas turbine aircraftengine; b) monitoring a set of parameters utilized by a control whichcontrols the engine; and c) if the set indicates that a predeterminedevent is occurring in the engine, moving a limit on temperature or speedaway from its normal position, and continuing to operate the engine. 13.A method, comprising: a) operating a gas turbine engine with limits oni) inlet temperature to the high-pressure turbine; and ii) speed of thehigh-pressure turbine; b) monitoring selected parameters of the engineand, if the parameters reach a predetermined state, raising either, orboth, of the limits.
 14. Method according to claim 13, wherein thepredetermined state indicates that actual thrust is less than 95 percentof demanded thrust.
 15. Method according to claim 13, wherein the limiton turbine inlet temperature is raised by 2.5 percent.
 16. Methodaccording to claim 13, wherein the limit on speed is raised by fourpercent.
 17. Method according to claim 13, wherein i) the predeterminedstate indicates that actual thrust is less than 95 percent of demandedthrust; ii) the limit on turbine inlet temperature is raised by 2.5percent; and iii) the limit on speed is raised by four percent.
 18. Amethod, comprising: a) operating a gas turbine engine in a normal mode,in a vehicle which carries a supply of fuel burned by the engine; b)monitoring behavior of the engine and, if a predetermined event occurs,consuming substantially all of the remaining operating life of theengine before the supply of fuel is exhausted.
 19. Method according toclaim 18, wherein substantially all of the remaining operating life isconsumed within 30 minutes of occurrence of the predetermined event. 20.A method, comprising: a) operating a gas turbine engine in accordancewith a first set of algorithms; b) obtaining a set of operatingparameters c) using a pattern recognizer to i) ascertain whether aproblem has occurred in the engine and, ii) if so, identify the problem;d) if the problem is identified, altering the set of algorithms. 21.Apparatus, comprising: a) a gas turbine engine; b) means for operating agas turbine engine according to a first control strategy; c) means fori) looking for damage to the engine; and ii) if damage is found,operating the engine according to a second control strategy. 22.Apparatus according to claim 21, wherein the second control strategyallows an operating parameter of the engine to exceed its normaloperating value.
 23. Apparatus according to claim 21, wherein the secondcontrol strategy allows an operating parameter of the engine to exceedits normal operating value by four percent.
 24. Apparatus according toclaim 21, wherein the second control strategy changes the schedule of acontrolled variable.
 25. Apparatus, comprising: a) a gas turbine engine;b) a control system which controls engine operation; c) means fordelivering measured operating parameters of the gas turbine engine tothe control system; d) means for i) inferring damage to the engine basedon some or all the operating parameters; and ii) is damage is inferred,taking one or both of the following actions: A) notifying an operator ofthe engine; B) altering control strategy of the engine.
 26. Apparatuscomprising: a) a gas turbine engine: b) means for maintaining a libraryof reference signatures of operating parameters, each of whichrepresents a respective damage condition in the engine; c) means for i)monitoring current operating parameters, and comparing them withreference signatures in the library; and ii) if the current operatingparameters match a reference signature, declaring a malfunction. 27.Apparatus, comprising: a) means for controlling a gas turbine engine ina manner that no operating parameters exceed pre-defined limits; b)means for i) examining operating parameters for a damage signatureindicating the existence of damage to the engine; and ii) if a damagesignature is found, controlling the engine so that at least one of theoperating parameters exceeds its limit.
 28. Apparatus, comprising: a)means for controlling a gas turbine engine in a manner that no operatingparameters exceed pre-defined limits; b) means for maintaining a libraryof reference damage signatures, wherein each signature indicates adifferent damage condition of the engine; c) means for maintaining acollection of control strategies, each strategy corresponding to adifferent damage condition; d) means for deriving a vector of operatingparameters from the engine; e) means for comparing the vector with thelibrary and, based on the comparison, i) determining whether a damagecondition exists, and ii) if so, selecting a corresponding controlstrategy and implementing it.
 29. Apparatus according to claim 28,wherein the selected control strategy causes at least one operatingparameter to exceed its limit.
 30. Apparatus according to claim 28,wherein the selected control strategy changes the control schedule of atleast one controlled variable.
 31. Apparatus according to claim 28,wherein the selected control strategy causes i) at least one operatingparameter to exceed its limit, and ii) a change in the control scheduleof at least one controlled variable.
 32. Apparatus, comprising: a) meansfor controlling a gas turbine aircraft engine; b) means for i)monitoring a set of parameters utilized by a control which controls theengine; and ii) if the set indicates that a predetermined event isoccurring in the engine, moving a limit on temperature or speed awayfrom its normal position, and continuing to operate the engine. 33.Apparatus, comprising: a) means for operating a gas turbine engine withlimits on i) inlet temperature to the high-pressure turbine; and ii)speed of the high-pressure turbine; and b) means for monitoring selectedparameters of the engine and, if the parameters reach a predeterminedstate, raising either, or both, of the limits.
 34. Apparatus accordingto claim 33, wherein the predetermined state indicates that actualthrust is less than 95 percent of demanded thrust.
 35. Apparatusaccording to claim 33, wherein the limit on turbine inlet temperature israised by 2.5 percent.
 36. Apparatus according to claim 33, wherein thelimit on speed is raised by four percent.
 37. Apparatus according toclaim 13, wherein i) the predetermined state indicates that actualthrust is less than 95 percent of demanded thrust; ii) the limit onturbine inlet temperature is raised by 2.5 percent; and iii) the limiton speed is raised by four percent.
 38. Apparatus, comprising: a) meansfor operating a gas turbine engine in a normal mode, in a vehicle whichcarries a supply of fuel burned by the engine; b) means for monitoringbehavior of the engine and, if a predetermined event occurs, consumingsubstantially all of the remaining operating life of the engine beforethe supply of fuel is exhausted.
 39. Apparatus according to claim 38,wherein substantially all of the remaining operating life is consumedwithin 30 minutes of occurrence of the predetermined event. 40.Apparatus, comprising: a) means for operating a gas turbine engine inaccordance with a first set of algorithms; b) means for obtaining a setof operating parameters c) means for using a pattern recognizer to i)ascertain whether a problem has occurred in the engine and, ii) if so,identify the problem and alter the set of algorithms, based on theproblem identified.